1. Field of the Invention
The present invention relates to an engine operation control technique in a series mode of a hybrid vehicle.
2. Description of the Related Art
Among recently developed hybrid vehicles, a vehicle has been known that allows a series mode in which a power generator is driven by an engine to generate electric power, and an electric motor causes travel driving.
In the series mode, only the electric motor drives drive wheels of the vehicle, and thus a rotational speed of the engine can be arbitrarily set irrespective of variations in traveling speed.
Generally, in the series mode, operation points (rotational speed, output torque) of an engine are set to values with low fuel consumption efficiency while ensuring an output (generated electric power) required by a power generator.
Further, a hybrid vehicle has been known in which an operation point of an engine in a series mode is changed. For example, Japanese Patent Laid-Open No. 2005-137135 proposes a technique of setting an operation point of an engine in a series mode based on a state of charge of a driving battery that supplies electric power to an electric motor, and maintaining the state of charge of the driving battery within a predetermined range.
However, in highlands with low atmospheric pressure, maximum output torque of an engine decreases. Thus, setting the operation point of the engine under conditions of flatlands (lowlands) may make it difficult to drive a power generator so as to ensure an output. Then, setting the operation point of the engine so as to ensure an output even in a highest altitude set in an area of use of a vehicle may unnecessarily increase a rotational speed of the engine in lowlands and thereby reduce fuel efficiency.